So mu is the matchmaking system’s best guess of how good you are. Sigma is how sure it is about that mu. Low sigma means more sure. So when you have a high sigma your mu changes much more per game, since the matchmaking wasn’t so sure about you.

You rating is mu - sigma * 3. This basically means that your rating is the worst possible guess about your bot’s real ability. Because of this, your rating climbs as the matchmaking system is more sure about your skill.